Aside from the fact that the statement is incorrect because as is evident in your post and others here people actually do eat horsemeat there is the problem of the meaning of, *Ο άνθρωπος" which means "*a human being, human, a person..." in general.
It does not mean "the human" as in a specific person but as a general meaning which is why a is required and for that reason there is no "the" option.
It's a bit like how the Wikipedia article on "lion" starts "The lion is a species ..." -- there, "the" does not refer to a particular lion, but to the idea of lions in general.
Similarly here, ο άνθρωπος could mean either "the human" (referring to a particular one) or to "humans; a (typical) human" in general.
Wouldn't the sentence 'The human does not eat horse meat' sound strange to you? Grammatically correct but weird? One would really have to come up with a contrived context to make sense of it, something like aliens trying to feed a captured human some horse meat. This is why the above Greek sentence is immediately understood to imply a general statment to a Greek speaker, even without any context.
Yes, "Ο" is the masculine article, however, the translation here does not call for an article which would indicate that a specific "human/person" eats horse meat" but it is the general expression of what occurs. Therefore, in English "a" is required or even plural. "Humans do not..."